A system and a receive-side circuit with the features mentioned there are known from B. Wedding, "New Method for Optical Transmission Beyond Dispersion Limit", Electronics Letters, 2nd Jul. 1992, Vol. 28, No. 14, pages 1298 and 1300. In that article, a system for optically transmitting a digital signal over a dispersive optical waveguide is described. The digital signal frequency-modulates an optical transmitter. As a result, wave trains with different wavelengths .lambda..sub.0 and .lambda..sub.1 are launched into the optical waveguide. Due to chromatic dispersion of the optical waveguide, the time required for light with the longer wavelength .lambda..sub.0 to propagate through the fiber-optic link is greater than that required for light with the shorter wavelength .lambda..sub.1. Accordingly, there is a period of time .DELTA..tau. during which the last part of the first wave train of wavelength .lambda..sub.0, for example, overlaps the first part of the wave train of wavelength .lambda..sub.1. This period .DELTA..tau. is called "propagation time difference" and is given by EQU .DELTA..tau.=.DELTA..lambda..multidot.D.multidot.L
where
.DELTA..lambda.=difference between .lambda..sub.0 and .lambda..sub.1 PA1 D=chromatic dispersion of the optical waveguide PA1 L=length of the fiber-optic link 3.
As a result of the above-mentioned overlaps, an intensity-modulated signal is received at the end of the fiber-optic link. To recover the digital signal from this signal, an integrator or low-pass filter in conjunction with a decision circuit is provided. The decision circuit is not explained, however.
It is, therefore, the object of the invention to provide a receive-side circuit whereby the digital signal can be recovered.
This object is attained by providing a receive-side circuit for a system that optically transmits a digital signal over an optical waveguide which is dispersive at the operating wavelength. The system includes an optical transmitter whose optical output signal, which is frequency-modulated by the digital signal, is optically transmitted over the optical waveguide. The input through the receive-side circuit is an electric output signal (V) for an optical-to-electrical transducer that passes through three ranges of values. A first of the ranges lies above an open threshold (V.sub.1). A second lies below a lower threshold value (V.sub.0). The third is between the upper and lower threshold values. The receive-side circuit recovers the digital signal from the sample values of the electric output signal (V) and is characterized by its output assuming a first state when a sample value lies in the first range of values, that its output assumes a second state when a sample value lies in the second range of values, and its output assumes the state which is present at the same output N-bit earlier than a sample value lies in the third range of values, where N is the rounded-up integral value of ##EQU1## D is the dispersion of the optical waveguide, L is the length of the optical waveguide, T is the duration of one bit, and .DELTA..lambda. is the wavelength difference underlying the frequency modulation.
One aspect of the invention is that based on the receive-side circuit, a novel optical transmission system is provided. The present invention, therefore, provides a system for optically transmitting a digital signal over an optical waveguide that is dispersive at the optical wavelength. The system includes an optical transmitter on the transmit side whose optical output signal is frequency-modulated by the digital signal. An optical-to-electrical transducer on the receive-side that converts its optical input signal, which passes through three ranges of values, into an optical output signal (V), that corresponds to the intensity variation of the optical input signal and passes through three ranges of values. A first of the ranges of values lies above the upper threshold value (V.sub.1), a second lies below a lower threshold value (V.sub.0), and a third lies between the upper and lower threshold values. The system further includes a circuit that follows the optical-to-electrical transducer which recovers the digital signal from the sample values of the electrical output signal (V) of the optical-to-electrical transducer. The circuit is characterized in that its output assumes a first state when a sample value lies in the first range of values, a second state when a sample value lies in the second range of values, and a state which is present at the same output N-bits earlier when a sample value lies in the third range of values, where N is the rounded-up integral value of ##EQU2## where D is the dispersion of the optical waveguide, L is the length of the optical waveguide, T is the duration of one bit, and .DELTA..lambda. is the wavelength difference underlying the frequency modulation. Further advantageous features of the invention are defined in the subclaims.
One advantage of the invention is that the receive-side circuit will recover the digital signal even if the propagation time difference .DELTA..tau. is a multiple of the duration of one bit.